Dry sift is when the dried flower heads are broken up and passed through a fine cloth or seive (sifted). The collected powder is then pressed into a block. Finer graded of seive offer different qualities of hash. The finer the mesh of the screen means less plant matter. And more trichomes.
There is no best hash to buy. It's all personal preference. I personally like dry sift as it does not involve any solvents.